The present invention relates to a sensor device that correctly detects the finished level of food cooked, for example, by a microwave oven.
Prior art
Conventional microwave ovens use absolute humidity sensors to detect the finished level of food cooked by radiating microwaves. FIG. 1 is a simplified electrical circuit diagram incorporating an absolute humidity sensor S, that contains a pair of thermistors S1 and S2 for detecting the humidity of exhaust gas from the cooking chamber containing the food. Thermistor S1 is installed so that it comes into constant contact with the exhaust gas. Thermistor S2 is placed in a dry atmosphere free of exhaust gas. Resistor Ra and variable resistor Rb are connected in parallel to the serial circuit composed of thermistors S1 and S2, respectively. When these thermistors are heated, the moisture in the exhaust gas adheres to thermistor S1, thus lowering the temperature of thermistor S1 by latent heat of water. Humidity in the exhaust gas can be detected by determining the difference in the resistance values between thermistor S1 which contacts the exhaust gas and thermistor S2 which is surrounded by dry air. When the humidity reaches a predetermined value, the microwave-generating magnetron stops operating. Conventional microwave ovens contain a pair of thermistors that form an absolute-humidity sensor positioned in the path of the discharged exhaust gas. The two thermistors so contained are equivalent in temperature to the exhaust gas. Accordingly, these two thermistors should contain identical temperature-dependent characteristics. In other words, the difference in the resistance values between these two thermistors should remain constant throughout the entire range of exhaust gas temperature. The temperature of the exhaust gas does vary widely from normal temperature up to 100.degree. C. In some cases, the sensor is operated in temperatures as high as 200.degree. or 250.degree. C. Therefore, it is quite difficult to develop a pair of ideal thermistors that have exactly the same temperature-dependent characteristics throughout the wide temperature range. Likewise, it is also difficult to develop thermistors which precisely hold resistance values at a constant level independent of temperature. This indicates that conventional methods can hardly provide satisfactory results. For example, when using the conventional circuit shown in FIG. 1, if the temperature-dependent characteristics of thermistors S1 and S2 are different from each other, the following problem may develop. If the exhaust gas temperature varies even though there is no humidity in the exhaust at all, as shown in FIG. 2, the judgment level "l" can be easily exceeded. In this case, the sensor will sense incorrectly that there is humidity in the exhaust gas. This will cause the microwave oven to stop operating at the wrong time.